1. Field of the Invention
The present invention relates to removal of vaporizable components from polymeric products. More particularly, undesirable vaporizable components are removed from biomedical polymeric products.
2. Description of the Related Art
Polymeric compounds are being used to form an increasing array of medical devices including sutures, drug delivery devices, textile-like materials such as bandages, tape and barriers, and other prosthetic devices. Such polymers include polypropylene, polystyrene, polycarbonate, etc. Bioabsorbable polymers are being used with increasing frequency in medicine because they can be implanted to perform a specified function and, when the function is complete, the polymer dissipates thus eliminating the need for further invasive surgical intervention.
Examples of bioabsorbable polymers used in medicine include polylactic acid, polyglycolic acid, polydioxanone, copolymers of glycolide and trimethylene carbonate, polylactide/polyglycolide copolymers, polyesteramides, etc. Polylactide/polyglycolide copolymers are frequently the bioabsorbable copolymers of choice because they retain strength after implantation in the body and slowly degrade into non-toxic breakdown products. Moreover, the copolymer may be molded or extruded into a variety of shapes.
During the formation of bioabsorbable polymers, some monomeric and/or dimeric moieties fail to polymerize and are formed in and/or deposited on the polymeric product. The monomer, dimer and other byproducts of the reaction process may have a detrimental effect on products made from bioabsorbable polymers, i.e., certain of these moieties weaken the polymeric products and interfere with their absorption properties. Such moieties are vaporizable and may be removed by application of heat. For example, U.S. Pat. Nos. 3,626,948 and 3,772,420 are directed to bioabsorbable sutures and a method of removing vaporizable impurities from such sutures by placing the sutures in an air-tight chamber and applying heat and vacuum. The impurities vaporize and diffuse to a cooled wall portion of the chamber and condense thereon. The devolatized suture materials can then be dyed, coated, needled, sterilized and packaged.
Another method of removing vaporizable impurities from polymeric products involves placing a polymeric product in a vacuum chamber having heated walls. An inert gas, such as nitrogen, is heated and introduced into the chamber from a single point source. The nitrogen acts as a carrier for the vaporized impurities which, along with the gas, is transported from the chamber.
The above-described processes for removing impurities have certain shortfalls which can lead to uneven heating of the polymeric product and prolongation of the removal process. The dynamics of heating chambers and various convection currents created therein result in uneven distribution of heat. The process disclosed in U.S. Pat. Nos. 3,626,948 and 3,772,420 involves heating a spool of suture material from a cavity within the spool. Uneven heating may result from a heat differential formed between the innermost and outermost portions of the spool.
In the above-described process involving nitrogen, and in accordance with the ideal gas laws, gas expands out of the single point source and cools. Such gas circulates unevenly around the chamber because gas located closer to the heated walls is kept or made hotter than the cooled gas elsewhere in the chamber. Moreover, heat from the chamber walls is not distributed evenly around the chamber due to convection currents created by the unevenly heated and unevenly distributed gas. Consequently, polymeric products are not heated evenly and vaporizable components may not be uniformly removed. In addition, the heating process is prolonged by the need to ensure that the entire product is heated sufficiently to vaporize all vaporizable components. Thus, a typical process may take as long as 50 hours or more, i.e., a first heating stage where the relevant atmosphere is brought up to sufficient heat, a second heat soaking stage, and a third cooling stage to bring the product to room temperature.
When the polymeric products are dyed prior to heat treatment, the volatile nature of the dye causes some dye to be removed through sublimation during the heat treatment. Uneven heat and gas treatment of the products is clearly revealed by a blotchy or mottled appearance of the heated product. This happens because the dye sublimes unevenly, i.e., in areas of greater heat, more dye sublimes and a lighter appearance is created than in cooler areas where less dye sublimes.
Consequently, the need to deliver a more uniform heat treatment to polymeric products undergoing post-treatment removal of vaporizable components is clear.